小菜成長記(十)——組合模式

public class Program {
    public static void main(String[] args) {
        Composite root = new Composite("root");
        root.add(new Leaf("Leaf A"));
        root.add(new Leaf("Leaf B"));

        Composite comp = new Composite("Composite X");
        comp.add(new Leaf("Leaf XA"));
        comp.add(new Leaf("Leaf XB"));

        root.add(comp);

        Composite comp2 = new Composite("Composite XY");
        comp2.add(new Leaf("Leaf XYA"));
        comp2.add(new Leaf("Leaf XYB"));
        comp.add(comp2);

        root.add(new Leaf("Leaf C"));

        Leaf leaf = new Leaf("Leaf D");
        root.add(leaf);
        root.remove(leaf);

        root.display(1);
    }
}

abstract class Component {
    protected String name;

    public Component(String name) {
        this.name = name;
    }

    abstract void add(Component component);
    abstract void remove(Component component);
    abstract void display(int depth);
}

class Leaf extends Component {

    public Leaf(String name) {
        super(name);
    }

    @Override
    void add(Component component) {
        System.out.println("Cannot add to a leaf");
    }

    @Override
    void remove(Component component) {
        System.out.println("Cannot remove from a leaf");
    }

    @Override
    void display(int depth) {
        for(int i = 0; i < depth; i++) {
            System.out.print("-");
        }
        System.out.println(name);
    }
}

class Composite extends Component {
    private List<Component> children = new ArrayList<Component>();
    public Composite(String name) {
        super(name);
    }

    @Override
    void add(Component component) {
        children.add(component);
    }

    @Override
    void remove(Component component) {
        children.remove(component);
    }

    @Override
    void display(int depth) {
        for(int i = 0; i < depth; i++) {
            System.out.print("-");
        }
        System.out.println(name);
        for (Component component : children) {
            component.display(depth + 2);
        }
    }
}

當你發現需求中是體現部分與整體層次的結構時，以及你希望用戶可以忽略組合對象與單個對象的不同，統一地使用組合結構中的所有對象時，就應該考慮用組合模式了。